The control of coypus (Myocastor coypus Molina) by cage trapping

Summary

Coypus Myocastor coypus had been feral in Great Britain (where now eradicated) since 1932 and by the 1950s were well-established in East Anglia. Damage to crops and to river and dyke banks was reported as their numbers increased. Trap-mark-release ('live trapping') experiments were carried out between November 1958 and May 1961 to assess the efficacy of cage traps, and to gain some information on coypu movements. When it became apparent that concerted action would be necessary to control numbers, an attempt was also made to systematically clear an area of coypus by cage trapping. The results of the live trapping and trapping-out work are described.

Study areas: Most trapping was carried out on marshland adjoining the River Yare in Norfolk, close to the city of Norwich. The first live trapping was undertaken at Hassingham, an area of grazing marshes interspersed with reed Phragmites australis beds and alder Alnus glutinosa and willow Salix spp. carr. The main live trapping was in similar adjoining habitat at Strumpshaw. Here a sand wall had been built to stop flooding of grazing marshes from an adjacent swampy area. An area of about 86 ha to the north of this wall was used for live trapping, most of this and an adjoining area to the south totalling about 127 ha was subsequently trapped out. In addition, live trapping was undertaken at Eaton on the River Yare, where coypus were feeding on a field of sugar beet.

Cage trapping: At first a cage trap employing a guillotine-type door was used but later a simpler trap with a swinging door was developed and used for subsequent live trapping and trapping-out. Coypus caught during the first live trapping and release experiments were marked by claw clipping but his was not entirely satisfactory as some claws are lost or broken in the wild. After February 1960, numbered ear tags were used, combined with claw clipping. Trapping was done over 4 nights/week, traps being examined once a day. On the other nights, traps were propped open and baited with root crops as available.

Most traps were set on well-used runs. An experiment in which alternate traps were camouflaged indicated that there was no advantage in concealing traps. In hot weather, traps were covered with cut reeds as coypus appeared to suffer heat stroke if exposed to hot sun.

The sand wall at Strumpshaw was crossed regularly by coypus and during live trapping at this site (February 1960 to May 1961), 29 traps were placed on the wall or by a dyke running parallel to it. Up to 18 further traps were used on sites up to 5 km from the wall, normally set in lines on the sides of dykes, on runs used by coypus when leaving the water.

Hassingham: North of the railway line that bisected the site, 47 coypus were trapped and marked; 32 were recaptured – 25 north of the railway. South of the railway 36 were caught and all but two of the 25 recaptured were recaught on the same side. This suggested that the railway acted as a partial barrier. Trapping continued until September 1959 and in 10 months 206 coypus was caught on about 4 ha. Four traps were placed on one corner of the site where a number of coypus were seen grazing one evening in August 1959. Fourteen previously uncaught coypus were subsequently caught over 3 weeks. Other traps set 200 to 400 m from these four traps since June, caught none of these 14 animals, suggesting that a group of coypus may be quite discrete and have only a small range. In January 1959 captures reduced by around 50% during very cold weather. It is probable that some coypus remained in cover during the severe cold spells, air temperature for this effect appearing to be a little below freezing.

Eaton: In 11 weeks 47 coypus (26 male, 21 female), were trapped. They were suspected of being recent immigrants exploiting a sugar beet crop; when the beet was harveted they left the area.

Strumpshaw: The 29 traps used were mostly kept in the same places and were only moved when it appeared that a more favourable run had developed. There were periodical high male:female sex ratios but not as marked as at Hassingham. Retrapping of marked animals suggested that their nightly range was unlikely to exceed about 200 m. Further information on their movements came from a marshman killing coypus who returned tags. Between February 1960 and 10 March 1961, 250 coypus were caught, of which only seven tags were returned from animals killed outside the live trapping area; all were males between 3.2-5.0 kg when last weighed. In addition to the animals trapped at Strumpshaw, 62 were tagged between December 1960 and March 1961 at Surlingham (about 1.6 km away). Only two of these were ever recovered, both at Stumpshaw; a 4.5 kg female caught December 1960 and a 5 kg male in November 1961.

Trapping of young: Very young coypus were not easily caught in cage traps. On some occasions, young would be caught in the same trap as the mother, in others the mother would be caught and the young would be outside the cage. Most young trapped were taken in some smaller mammal traps used.

Trap baiting: Baiting did not normally increase trapping success, e.g. in one comparison where alternate traps were left unbaited, 23 of 34 (68%) baited traps and 24 out of 36 unbaited ones(67%) caught coypus.

Conclusions: There appeared to a more or less continuous interchange of some animals between neighbouring populations. It appears that females probably move fairly short distances before littering whilst males move longer distances. At peak littering times trapping may catch twice as many males as females. When individuals were moved from their place of trapping they quickly moved back again upon release. An area was trapped out and from this it was concluded that coypus can be eliminated from an area using cage traps if immigration can be prevented. The experience gained formed the basis for the operations of the Government-sponsored Coypu Campaign (August 1962 to December 1965).